Question: On the first 7 quizzes of her physics class, Emily got an average score of 78. What does she need on the next quiz to have an overall average of 80?
Answer: Let her score on the next quiz be $x$ The sum of all of her scores is then $7 \cdot 78 + x$ The same sum must also be equal to $8 \cdot 80$ Solve: $x = 8 \cdot 80 - 7 \cdot 78 = 94$.